1. Field of the Invention
The present invention relates to oil and gas well proppants and, more particularly, to sintered proppants made from ingredients at least some of which are uncalcined or partially calcined, a method of making such proppants, and to a method of maintaining a fracture in a subterranean formation in a propped condition by utilizing such proppants.
2. History of the Prior Art
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped from the well. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure, and the proppant material or propping agent is a particulate material, such as sand, glass beads or ceramic pellets, which are carried into the fracture by means of a fluid.
Spherical pellets of uniform size are believed to be the most effective proppants due to maximized permeability. For this reason, assuming other properties to be equal, spherical or essentially spherical proppants, such as rounded sand grains, metallic shot, glass beads and tabular alumina, are preferred.
In practice, in deep wells, where high pressures are encountered, e.g., above about 700 kg/cm.sup.2 (10,000 psi), the immediately foregoing specifically mentioned proppants are either entirely ineffective or do not exhibit desired permeability. Examples of prior art proppants and their use are found in U.S. Pat. Nos. 2,950,247, McGuire, et al; 3,026,938 Huitt, et al; 3,126,056, Harrell; 3,497,008, Graham, et al; 3,976,138, Colpoys, et al; and 4,068,718, Cooke, et al. One of the better proppants useful at high pressures, disclosed in U.S. Pat. No. 3,976,138, is predominantly alumina. However, even such alumina, as disclosed in U.S. Pat. No. 3,976,138, has reduced permeability at pressures in excess of 350 Kg/cm.sup.2 (5,000 psi).
As disclosed in U.S. Pat. No. 4,068,718, sintered bauxite made from calcined bauxite unexpectedly has a permeability which is superior to the previously mentioned proppant materials at pressures as high as 700 kg/cm.sup.2 (10,000 psi) or higher. Pellets having a high apparent specific gravity, i.e. greater than 3.4, are disclosed in U.S. Pat. No. 4,068,718 to be most suitable as proppant materials.
The prior art sintered bauxite particles made from calcined bauxite may be produced in spherical shape as described in R. J. Seider's commonly assigned, copending U.S. patent application Ser. No. 252,491, filed Apr. 8, 1981, as a continuation of U.S. patent application Ser. No. 969,122, filed Dec. 13, 1978. Such prior art sintered bauxite proppants prepared from fully calcined bauxite, although extremely useful under high pressure conditions, over about 700 kg/cm.sup.2 (10,000 psi), are costly. The cost of the prior art high strength, sintered, calcined bauxite proppant for wells of intermediate pressures, between about 350 and 700 kg/cm.sup.2 (5,000 and about 10,000 psi), may not be economically justified.
The manufacture of sintered spherical pellets from calcined clay and calcined bauxite, calcined alumina or mixtures thereof, is described in J. F. Fitzgibbon's commonly assigned, copending U.S. patent applications, Ser. Nos. 347,210 filed Feb. 9, 1982, and 405,055 filed Aug. 4, 1982. These pellets are available at somewhat lower cost and are aptly suited to use under pressures of up to about 700 Kg/cm.sup.2 (10,000 psi). These pellets have lower specific gravities and bulk densities than those made according to U.S. Pat. No. 4,068,718.
Calcining adds considerably to the cost of the raw materials or ingredients used in the manufacture of pellets useful as proppant. For example, the cost of dried diaspore clay is about 35 dollars per ton, if air dried. The cost of the same material is about 70 dollars per ton, if fully calcined.
While the prior art ceramic pellets made from fully calcined ingredients are aply suited for many proppant applications there remains a need to provide strong ceramic pellets that are of even lower cost.
The present invention provides strong pellets aptly suited to use as proppants under pressures of up to about 700 kg/cm.sup.2 (10,000 psi), which are more economical than previously available synthetic ceramic proppants and have low specific gravities and bulk densities, which benefit the user, in that fewer pounds of proppant per cubic foot of fracture are required. Handling, e.g., pumping of slurries of low density material, is easier than handling of high density materials.